Grape Maturation Grape Maturation and Ripening Changes occurring during the ripening process: Following bloom and fruit set, grape berries undergo a rapid phase of growth by cell division that transitions to growth by cell enlargement. As the berries begin to touch, growth slows that is referred to as the lag phase, and the process of maturation begins at veraison (Figure 108). Several changes occur during veraison: Skin color of colored cultivars changes from green to red, blue or black (Figure 109A). Berries begin to soften, with white cultivars becoming more translucent (Figure 109C). Sugars (measured as oBrix or %SS) begin to increase (Figure 110). Acids [measures as titratable acidity (TA)] begin to decrease (Figure 110). Juice pH begins to increase (Figure 110). Figure 109. Veraison in a red wine cultivar (A). Lag Figure 108. Grape berry development & phase (B) and veraison (C) in a white wine maturation. (Illustration by J. Koutroumanidis, Winetitles) cultivar. Figure 111. Pedicel/berry abscission zone: Figure 110. Changes in oBrix, titratable acidity (TA) immature (A), mature (B). Seed maturity: and initial pH following veraison. immature (C), mature (D). Juice color begins to change. White’s – greenish to whitish. Red’s – begin to take on some skin pigment. Skin tannins begin to polymerize to become more desirable. Seed tannins (undesirable) begin to become less extractable. Varietal flavor components begin to increase. The rachis begins to mature. An abscission zone between the pedicel (berry stem) and berry begins to develop (Figure 111A, B). Grape Maturation Seeds begin to mature (Figure 111C, D). As these changes occur, the transport system to the berries changes from xylem transport of mostly water associated with berry enlargement to phloem transport of mostly sugars as the berries enter the engustment (ripening) phase. Testing grape maturity: Following veraison, you should begin testing the maturity of the grapes to determine when to harvest, and there are several tests that can be used to monitor the changes that occur during the maturation process. Sugars: The concentration of sugars, expressed as either oBrix or % SS, increases during maturation and can be measured with a refractometer or a hydrometer. The Brix refractometer measures the degree sugar molecules bend light as it passes through a prism with the degree of bending being associated with the concentration of sugars. The hydrometer measures the specific gravity of the sugars (soluble solids) in the grape juice. The refractometer is easy to use and can be taken into the field to sample berries, while the hydrometer requires enough juice to be floated. Sugars are important for wine making because to 1% sugar converts to 0.55% alcohol (% SS x .55 = % alcohol). Sugars are easy to measure and are a common measurement in the wine industry, but it is not a good measure of maturity when used alone. This is particularly true for some of the Northern hybrids such as Frontenac, La Crescent and Marquette that are high in acids and can have high concentrations of sugar shortly after veraison. In addition, there is a poor relationship between sugar levels and accumulated berry flavors (Wolf, editor, 2008). Initial Juice pH: During maturation, juice pH increases and is measurable with a pH meter. Juice pH is important because wine balance or the perception of sourness or tartness, aroma, microbial stability, and physiochemical stability of the wine. When grapes harvested at a below optimal juice pH, the wines tend to be sour, herbaceous and lack character. At a pH above 3.5 to 3.6, wines may be “flat” in character, more prone to microbial infection, more oxidized and have less color, and have less desirable flavors (Wolf, editor, 2008). Titratable acidity (TA): During maturation the organic acids in the juice decline, and this can be measured as titratable acidity. It requires a pH meter to measure the amount of a strong base [0.1 N sodium hydroxide (NaOH)] to neutralize the juice to an end-point of 8.2 pH for tartaric acid. Titratable acidity is expressed as either the percentage acidity (% TA) or grams of acid per liter where 1 % TA is equivalent to 10 g/liter TA. Titratable acidity serves as a measure for developing a sugar/acid balance of the juice and determines how sweet a wine needs to be finished to maintain a good balance (Wolf, editor, 2008). The oBrix, initial juice pH and TA are measurable indicators of maturity and when used together, provide good guidance in determining when to harvest, and optimum maturity indices have been developed for different styles of wine (Table 39). Table 39. Optimal juice oBrix, initial pH and titratable acidity (TA) for different styles of wine. oBrix Initial TA Wine Style (% SS) pH (g / liter) White table wine z 21 - 22 3.2 – 3.4 7 – 9 Red table wine z 22 - 24 3.3 – 3.5 6 – 8 Sparkling wine y 17 - 20 2.8 – 3.2 7.0 – 9.0 White table wine y 19 - 23 3.0 – 3.3 7.0 – 8.0 Red table wine y 20 – 24 3.2 – 3.4 6.0 – 7.5 Sweet table wine y 22 – 25 3.2 – 3.4 6.5 -8.0 Dessert wine y 23 – 26 3.3 – 3.7 5.0 – 7.5 z Dharmadhikari and Wilker. 2001. Micro Vinification, a practical guide to small-scale wine production. y Wolf, editor, 2008. Wine Production Guide for Eastern North America. Grape Maturation These are optimal indices, but for Northern hybrids that are characteristically high in acids, grapes are typically harvested before the TA drops into the optimal range. Often the initial pH is allowed to approach the upper end of its optimal range to allow the TA to come down to a reasonable level. Other tests for measuring changes that occur during the grape maturation process are available and can aid in determining when to harvest. However, they are subjective measurements, and often require several years of experience to master. These measures include: Skin tannins: As grapes mature the skin tannins polymerize from short chain to long chain molecules. With practice, this change can be detected on the tongue, particularly with red cultivars. Un-polymerized “unripe” tannins cause friction in the mouth when they bind with salivary proteins, while polymerized “mature” tannins cause less binding with the salivary proteins. To test for skin tannins, separate the skins form the berry pulp and chew it to release the tannins. Then press and move your tongue against the roof of your mouth (palate). If the tongue sticks or binds against the palate, the tannins are “green” or gripping and less mature. If the moves freely and “slides”, the tannins are judged smooth or “silky” and more mature (Wolf, editor, 2008) Berry detachment: As grapes mature an abscission zone begins to form between the pedicle and berry, and ease of berry detachment can serve as an indicator of maturity. When detaching a berry from a cluster, observe where the pulp and skin detach cleanly from the pedicel. If some pulp and/or skin remain attached to the pedicel (Figure 112A), the berry is considered un-ripe. If the no pulp remains attached to the pedicel (Figure 112B), the berry is judged to be ripe (Wolf, editor, 2008). Once the abscission zone is formed, the berries will not mature any further, but can become sweeter from of dehydration. For cultivars such as La Crescent that are prone to shelling, checking for the beginning of berry detachment is an important procedure for determining when to harvest. Seed maturity: As grape seeds mature, their color changes from green to brown to dark brown. This color change is associated with oxidative reactions and corresponds with the decrease in extractable seed tannins. Seed color has been used as an indicator or maturity, but uniform and complete brown seed color is seldom achieved in Eastern and Midwestern vineyards (Wolf, editor, 2008). Aroma and Flavor: Both aroma and flavor can be used as indicators of grape maturity. With training human sensory skills can be developed to recognize flavor, aroma, sweetness and acidity development that characterize the maturation of a particular grape cultivar. However, it takes years of experience to develop these skills. It is one of the few ways to assess “balance” (Wolf, editor, 2008). Table 40 summarizes these maturity tests that can be conducted for determining when to harvest and provides guidance regarding there use. Table 40. Harvest parameters that can be performed to determine when to harvest wine grapes. Run the Harvest parameter test? Comments oBrix Yes Test along with initial pH and TA. Initial juice pH Yes Test along with oBrix and TA. Titratable acidity Yes Test along with oBrix and initial pH. Skin tannins Check Use along with oBrix, initial pH and TA. Berry detachment Check Use along with oBrix, initial pH and TA as an indicator. Along with oBrix, initial pH and TA, learn to develop the Aroma Maybe skill to detect changes for each cultivar. Along with oBrix, initial pH and TA, learn to develop the Flavor Maybe skill to detect changes for each cultivar. Seed color Maybe Use along with oBrix, initial pH and TA as an indicator. Grape Maturation Collecting a sampling to test for maturity: Sampling to test for maturity should begin following veraison. Initially it can be done in the vineyard with a refractometer to determine when to begin collecting a larger sample to run a more through test for oBrix, juice pH and TA. When collecting a sample, it is very important to collect a sample that is representative of the crop. Grape maturity is not uniform between vines, within vines and within grape clusters.